Casino owners, labor and other interests spent more than $3.6 million to influence lawmakers during last month's special session to expand gambling in Maryland — a figure that comes to about $900,000 a day for the four-day session, according to disclosure reports filed Monday.

The biggest spender was a Washington-area labor organization which — with help from the owners of the National Harbor development — poured $2.7 million into television advertising designed to persuade the General Assembly to pass legislation authorizing table games and a new casino in Prince George's County.

The total reported by the Washington Building & Construction Trades Council, an umbrella organization of labor unions, was far greater than any other company or organization that had reported its spending by Monday afternoon. The council's spending was done in the name of Building Trades for National Harbor.

"It's a staggering number. It's not a surprise, given what's at stake here," said James Browning, Mid-Atlantic director of the watchdog group Common Cause. He said the operators of a new casino could realize enormous profits, while existing casinos could be hurt by new competition.

Question 7 on the November ballot is now the subject of a high-stakes ad war between backers of National Harbor as the site for a sixth casino and Penn National Gaming, owner of the Hollywood casinos in Perryville and Charles Town, W.Va.

The requirement that groups disclose their lobbying and other spending to influence the special session's outcome was included in the casino legislation passed by the General Assembly and signed by Gov. Martin O'Malley.

The ad blitz has continued as both sides seek to sway voters who will cast ballots this fall. The money earmarked for advertising by the two sides since the session totals about $15 million — making it likely that spending on the referendum will eclipse the $17 million spent by the O'Malley and Ehrlich campaigns during the 2010 race for governor.

The building trades council is supporting an effort by MGM Resorts International to build a "destination" casino at National Harbor on the Potomac River — a project it believes will generate thousands of high-paying construction jobs. Meanwhile, the Peterson Cos., developer of National Harbor, disclosed that it had contributed $591,897 to Building Trades for National Harbor.

The council's disclosure was part of a filing by its lobbyist, Mark Coles, who did not return calls seeking comment.

On the other side, groups opposing the legislation disclosed Monday that they spent more than $600,000 before and during the special session on their media campaigns against the measure.

Meanwhile, dozens of Maryland lobbyists filed disclosures showing that they collectively earned more than $320,000 for representing clients with an interest in the gambling bill during the special session.

The totals will almost certainly grow, because some groups and lobbyists were filing right at Monday's deadline or may have mailed in their forms.

A group representing minority contractors in Prince George's County reported spending $289,317 on an ad campaign opposing the legislation. The Prince George's Contractors and Business Association trained its fire on MGM Resorts International, the prospective operator of a casino at National Harbor, but the group said its real dispute was with the Peterson Cos. over minority contracting at National Harbor.

A less obvious player was the National Gay and Lesbian Task Force Action Fund, which reported spending $346,938 on casino-issue mailings — not a topic on which it might be expected to have a firm position.

Rea Carey, executive director of the fund, said the spending was for a mailer urging lawmakers to oppose the bill to keep the issue of gambling off the November ballot. Carey said the group was concerned that a gambling referendum could have an adverse impact on a separate ballot question involving same-sex marriage.

"There were concerns that having the gaming measure on the ballot could jeopardize the outcome of the marriage equality vote, which was the focus of the mailer," Carey said in a statement. "This has always simply been about ensuring that nothing gets in the way of securing marriage equality in Maryland."

It was not clear whether all the money spent on advertising for or against the gambling measure has been disclosed. For instance, the Taxpayers Protection Alliance, a Virginia -based group that mounted a TV ad campaign against the legislation, reported spending only $1,000.

For some lobbyists who were paid an annual retainer, the special session brought additional work and not much money. But for some, the high-stakes struggle among well-heeled interests made for a lucrative summer.

Members of the firm Capitol Strategies, for instance, earned more than $58,000 from a variety of clients, including bingo parlor operators and MGM.

Peterson disclosed that its had paid Bruce C. Bereano $40,000 and Tim Perry $20,000 for their services.

The Cordish Cos., owner of the Maryland Live Casino at Arundel Mills, paid company officials David Cordish, Blake Cordish and Joseph Weinberg $5,000 each for their efforts. Somewhat more expensive were their paid lobbyists, Marta Harting, John Stierhoff and Brian Quinn, each of whom reported receiving $6,666, and Carville Collins, $6,200. Earning the most from Cordish was J. Steve Wise, with $17,500.

Todd Eberly, a professor of political science at St. Mary's College of Maryland, said the short session made it all the more important for interests groups to hire lobbyists who could get them access to lawmakers.

"Given the nature of the session, you either spend money or you're drowned out," he said. "If you want a member of the Assembly to hear you, you'd better be ready to write a check to somebody."

The lobbying disclosures are separate from those required for spending on the referendum campaign. Ballot committees must report contributions or expenditures of more than $10,000 to the State Board of Elections within 48 hours.

As of last week, the campaign committee bankrolled by MGM had received $5.4 million to support the ballot question, while the group formed by Penn National Gaming to oppose it had collected $9.5 million.

The legislation that passed last month would permit table games at all of Maryland's three opened and two planned casinos and create a sixth license for Prince George's County. While the measure provides for a competition between National Harbor and Penn National's Rosecroft Raceway for that license, Penn National contends the process has been stacked against it.

Penn National also stands to lose revenue at its Charles Town casino if a competitor comes to National Harbor and draws customers from Washington and Northern Virginia.